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Module 1
Refuse
Refuse is a general term used to indicate what is rejected or left out as worthless. It may be in liquid, semi-solid, or solid form and many be divided into six categories: garbage, rubbish, sullage, sewage, subsoil water and storm water.
Garbage
Garbage indicates dry refuse. It includes waste paper, decayed fruits and vegetables, grass and leaves, and sweepings from streets, markets and other public places. Thus garbage contains large amount of organic and putritying matter.
Rubbish
Rubbish indicates sundry solid wastes from offices, residences and other buildings. It also indicates waste building materials, broken furniture, paper, rags etc. Generally, rubbish is dry and is of combustible nature.
Sullage
Sullage is a term used to indicate the wastewater from bath rooms, kitchens, washing places and wash basins etc. it does not create bad smell since organic matter in it is either absent or is of negligible amount.
Sub-soil water
It is the ground water that finds it entry into sewers through leaks.
Night soil
It is a term used to indicate the human and animal excreta.
Sewage
Sewage is the liquid waste produced from a community generally conveyed by a sewer. It includes sullage, discharge from latrines, urinals, stables industrial waste and also the ground surface and storm water that may be admitted into the sewer. It is extremely putrescible; its decomposition produces large quantities of malodorous gases, and it may contain numerous pathogenic or disease producing bacteria.Depending upon the nature and source, sewage is classified as:
- Domestic Sewage
- Industrial Sewage
1. Domestic Sewage
Domestic sewage is that which originates in the sanitary conveniences of a dwelling, business house or institution.
2. Industrial Sewage
It is the liquid waste produced from industrial process, as dying, brewing, paper making etc;
3. Sanitary Sewage or Dry Weather Flow
Combination of domestic sewage together with industrial waste is termed as sanitary sewage or DWF. the quantity of DWF is determined by considering the following factors: ● Infiltration and exfiltration ● Nature of industries ● Population rate of water supply
Infiltration:- It is used to indicate the leakage of water from the ground
surrounding the sewer.
Exfiltration:- It is used to indicate the leakage of sewage from the sewer into the
ground surrounding the sewer
Sources of sanitary sewage
★ Water supplied to the public for domestic purposes by the local authority ★ Water supplied to the various industries for various industrial processes by the local authority ★ Water drawn from wells by individual houses for their domestic purposes ★ Water supplied by the local authority to various public places such as schools, cinemas, hotels, railway stations etc; ★ Water drawn from the wells, lakes, canals etc by industries for their purposes ★ Infiltration of groundwater into sewers through leaky joints ★ Unauthorised entrance of rain water in sewer lines
Sewerage system
Nature of sewers provided for carrying different types of liquid waste will indicate the sewerage system.
Determination of quantity of sanitary sewage
Quantity of sanitary sewage = Total quantity of water supplied + Additions due to industries - Subtractions The additions are done due to private water supplies and infiltration of water. The subtractions are done due to leakage of water in pipe lines and water being consumed in drinking, cooking, sprinkling in roads, gardening of lawns and gardens converting into
Rate of use of water and the consequent rate of sewage production increases with increase in population. This is due to the fact that increase in the population of the town is mainly due to improved facilities.
3. Type of area served: The quantity of wastewater produced depends upon whether the area to be served is residential, commercial or industrial. The wastewater from the residential area directly depends upon the rate of water supply. If there is no infiltration of water in the sewers, and if there are no private source of supply, the wastewater produced from the residential area may be assumed to be equal to 70 - 80% of the water supplied through the public supply system. The amount of wastewater produced from the industrial locality depends up on the type of industries and their corresponding industrial processes. 4. Infiltration of subsoil water or groundwater: Ground water or subsoil water may infiltrate into the sewers through the leaky joints. Due to infiltration the quantity of flow through sewer increases, exfiltration results in decrease in the flow and consequent increase in the pollution of ground water. Infiltration of water into the sewer depends upon the following factors: ❏ Depth of sewer below ground water level ❏ Size of sewer ❏ Length of sewer through infiltration prone zone ❏ Nature and type of soil through which sewer is laid ❏ Type of joints, workmanship etc, ❏ Sewer materials
Storm water
Storm water is the liquid flowing in sewers during or following a period of rainfall. It indicates the rain water of the locality. The quantity of storm water which is known as Wet Weather Flow (WWF)(Peak runoff). Storm water flow will depend on the following factors: ● catchment area ● Ground slope ● Permeability of the ground ● Extent of impervious area ● Extent of vegetation growth ● Rainfall intensity ● Rainfall duration ● Condition of ground prior to rainfall ● Concentration or compactness of the catchment area
● Climatic conditions( wind, humidity, temperature etc;)
1. Time of concentration
Time of concentration of a drainage basin may be defined as the time required by the water to reach the outlet from the most remote point of the drainage area. In other words, it is the time required for the flood discharge to reach to the max. Limit.
(a) Inlet time
It is the time taken by the water to flow over land from the critical point upto the point where it enters the drain mouth. The time of entry decreases with increase in slope and imperviousness of the ground, while it increases with distance and storage conditions of
K1 = 0.9, K2 = 0.85, K3= 0.8, K4 = 0.4, K5 = 0.1, K6=0. A1=20% of Total area = 20% of A, A2 = 20% of A, A3 = 5% of A, A4 = 15% of A, A5 = 35% of A, A6 = 5% of A
Hydraulic design of sewers
Hydraulic formula for determining flow velocity in sewers:
Design of sewers is done on the basis of following hydraulic formula:
1. Chezy’s formula
V = Velocity of flow in the channel in m/sec R = Hydraulic mean radius of channel or Hydraulic mean depth of channel = A/P A = area of channel P= Wetted perimeter of the channel S = Hydraulic ground = ground slope for uniform flows, ie, the head drop between the two points divided by the length C = Chezy’s formula, the value of C can be obtained using either the Kutter’s Formula or the Bazin’s formula Q = A x V A = Flow area of cross section V = Flow velocity in the channel
Kutter’s formula:
𝐶 = 23+
( (^) 𝑆 )
1 𝑛 1+ 23+
( (^) 𝑆 ) 𝑛 𝑅 R= hydraulic mean depth = A/P, N = Rugosity coefficient, n = 0.013 for sewer pipes
Bazin’s formula:
𝐶 =
1.81+ 𝐾 𝑅 K = Bazin’s constant = 0.3 for smooth concrete lined surface
2. Manning’s formula
𝑉 = 1 𝑁 𝑅 2 3 𝑆 1 2 S = Bed slope of the sewer, R = Hydraulic mean depth , N = Rugosity Coefficient
3. Crimp and Burge’s formula
Minimum velocity in sewers = 0.45 m/sec (Self cleansing) Average velocity = 0.9 m/sec For self cleansing , 𝑆 𝑠 = 𝑅 ( (^) 𝑟 ) 𝑆 𝑉 𝑠 𝑉 = 𝑁 𝑛 𝑟 ( (^) 𝑅) 1 6 𝑞 𝑠 𝑄 = 𝑁 𝑛 𝑞 𝐴 𝑟 ( (^) 𝑅) 1 6
